1. Field of the Invention
The present invention generally relates to a development device using two-component developer consisting essentially of toner and carrier, a process cartridge including the same, and an image forming apparatus, such as a copier, a printer, a facsimile machine, or a multifunction machine having at least two of these capabilities, that includes the same.
2. Description of the Background Art
Two-component developer consisting essentially of toner and carrier is widely used in electrophotographic image forming apparatuses.
Development devices using two-component developer typically include a developer container in which developer is contained, a rotary developer carrier such as a development roller, and a developer conveyance member such as a conveyance screw provided in the developer container. The developer conveyance member supplies developer to the developer carrier while transporting the developer through a developer supply compartment (i.e., a developer supply path) in the developer container in an axial direction of the developer carrier. Then, the developer carrier rotates and supplies the developer carried thereon to a development area facing an image carrier such as a photoconductor. After having passed through the development area and toner therein has been consumed, the developer (hereinafter “used developer”) is collected either in the supply compartment (hereinafter “one-conveyance path method”) or a collection compartment separate from the supply compartment (hereinafter “supply-collection separation method).
The one-conveyance path method has a drawback in that the concentration of toner in the developer in the supply compartment decreases downstream in a direction in which the developer is transported (hereinafter “developer conveyance direction”), and accordingly the concentration of toner in the developer supplied to the development area is uneven in the axial direction of the developer carrier. Such unevenness in toner concentration causes unevenness in image density of images formed on sheets of recording media and is undesirable. It is to be noted that hereinafter “downstream” and “upstream” as used in this specification means downstream and upstream in the developer conveyance direction unless otherwise specified.
To solve the problem described above, for example, JP-2002-006599-A employs a supply-collection separation method in which used developer is collected in a collection compartment separate from the supply compartment (hereinafter “a supply-collection separation-type development device”). In the supply-collection separation-type development device, the concentration of toner in the developer in the supply compartment can be kept substantially constant in the developer conveyance direction. Although the concentration of toner in the developer supplied to the development area can be kept uniform in the axial direction of the developer carrier in the supply-collection separation-type development device, doing so makes it impossible to make such development devices compact. At present, it is preferred to make development devices more compact to make the image forming apparatus incorporating the development device compact.
Several approaches, such as those described below, have been tried to make supply-collection separation-type development devices compact. For example, a vertical agitation arrangement shown in FIG. 17 is effective to make supply-collection separation-type development devices compact. More specifically, in a known supply-collection separation-type development device 3Z shown in FIG. 17, a supply compartment 301SZ is disposed adjacent to and to one side of a developer carrier 302Z (in FIG. 17, in a lateral direction), and a collection compartment 301CZ is disposed beneath the supply compartment 301SZ. The collection compartment 301CZ receives developer that has been transported to a downstream end portion of the supply compartment 301SZ. In the development device 3Z, developer is circulated in the direction indicated by outlined arrows A1 and A2 shown in FIG. 17.
However, the vertical agitation arrangement has a limitation regarding the location of a toner supply mechanism for supplying toner to the development device. In the development device 3Z shown in FIG. 17, developer is transported from the downstream end portion of the supply compartment 301SZ to an upstream end portion of the collection compartment 301CZ and transported from a downstream end portion of the collection compartment 301CZ to an upstream end portion of the supply compartment 301SZ, thus forming a developer circulation path.
In the development device 3Z, as the location of a toner supply port 310Z through which toner is supplied to the developer circulating in the developer circulation path, a portion of the supply compartment 301SZ facing the development area of the developer carrier 302Z must be avoided. If toner is supplied to that portion, the toner just after supplied is likely to be carried by the developer carrier 302Z to the development area. Since electrical charge of the toner just after supplied is insufficient, such toner can scatter on the backgrounds of output images or around the interior of the image forming apparatus if being used in image development.
In view of the foregoing, it is preferred that the toner supply position be positioned further from the upstream end portion of the supply compartment 301SZ on the developer circulation path, outside the portion facing the development area of the developer carrier 302Z. Such an arrangement can increase contact between the supplied toner and carrier particles in the developer, thus charging the supplied toner better, before the supplied toner reaches the upstream end portion of the supply compartment 301SZ where the developer is carried onto the developer carrier 302Z.
Therefore, in supply-collection separation-type development devices, toner is typically supplied to the collection compartment 301CZ. However, to supply toner to the collection compartment 301CZ in the vertical agitation arrangement in which the collection compartment 301CZ is positioned beneath the supply compartment 301SZ, the arrangement is limited to the two arrangements described below.
In a first arrangement, as shown in FIG. 17, a toner supply route 350 is positioned on the side of the supply compartment 301SZ, opposite the developer carrier 302Z, and the toner supply port 310Z, which is at a downstream end of the toner supply route 350, is formed in a side wall of the collection compartment 301CZ positioned beneath the supply compartment 301SZ. This arrangement increases the size of the development device 3Z in the lateral direction in FIG. 17 by a length corresponding to the width of the toner supply route 350.
FIG. 18 illustrates a second arrangement regarding the location of the toner supply route. As shown in FIG. 18, in a development device 3Z1, a collection compartment 301CZ′ is made longer than a supply compartment 301SZ′ in the axial direction of a developer carrier 302Z, and a toner supply port 310Z is provided in the expanded portion of the collection compartment 301CZ. This arrangement increases the size of the development device 3Z1 in the axial direction of the developer carrier 302Z (in FIG. 18, the lateral direction) by a length corresponding to the width of the toner supply port 310Z.
As described above, in the vertical agitation arrangement in which the collection compartment is disposed beneath the supply compartment, the development device becomes bulkier when toner is supplied to the collection compartment, which is not desirable.
In view of the foregoing, the inventors of the present embodiment recognize that there is a need for the development device to reduce scattering of toner in the background of output images and around the interior of the image forming apparatus without increasing the size of the device.